Re: wire guided vehicle

Greetings:

The term wire guided is used several ways. There are wire guided
torpedoes, wire guided missiles and wire guided fork lifts etc. for
warehousing use. Your question relates to the last category which often
is called INDUCTIVE GUIDANCE because the robot vehicle follows the
magnetic induction field from an energized wire. This should not be 
confused with magnetic guidance which uses magnets along a path to guide a 
robot.

In one of our factories we have mail delivering robots that circle the
factory floor several times a day , entering offices etc. The robots use 
photocells to follow a white line painted on the floor. The mail robot 
operates in a similar way that the induction guided vehicles do. Two photo 
cells are mounted on the front of the robot about 2.5 cm (1 inch) above the 
floor. The photocells are spaced about 5 cm (2 inches) apart and straddle 
the 2.5 cm (1 inch) wide white line on the floor. The photocells are wired 
so that the right photocell generates a negative voltage over the white 
line and the left photo cell generates a positive voltage when over the 
white line. When the photocells are equally spaced from the line, (each 
about 1/2 way over the line)  their two equal and opposite voltages cancel 
each other. If the robot moves off to the right a increasing positive 
steering voltage is generated which steers the robot left and if the robot 
is off to the left an increasing positive voltage is generated to steer the 
robot right. Thus by steering the photocell voltages to zero, the robot 
follows the line. There are a number of toy robots for
sale in electronics stores and science shops that use this type of 
guidance.

The inductive guidance system follows the electromagnetic field from an 
energized buried wire in a manner similar to the photocell robot. Two pick 
up coils (antennas) are placed near the ground in front of the robot with 
the axis of the coils parallel to the ground and normal (at right angles) 
to  the direction of the wire. The antennas are wired to produce opposite
currents (and voltages) generating a steering signal in a manner similar to
the photocell robot.

There are a number of companies that sell these types of systems and
robotic vehicles that follow buried wires for distances as great as 300
meters (about 1000 feet) and the Swedish AVG Electronics has diagrams and 
pictures of their equipment. antennas etc. at the following web site:

http://www.agve.se/wire.htm AGV Electronics

Other web sites are: 

http://www.pmh-co.com/wireguid.html 

and http://www.mlr.de/einduktiv.htm

The AGV web site has the best information that I could find; however, they 
do not say what frequency signal that they use for the wire. They do 
mention that they have cross wires with different signals to tell the 
robots where to stop and to give distance marks.

Recently I had to find some water sprinkler valve control wires that ran 
about 100 feet and were buried about  30 cm (1 foot) under my lawns by a 
previous owner of the property. I connected a tunable radio frequency 
signal generator (100KHz to 20 MHz) to the control wires at the sprinkler 
control box and used my Sony transistor radio (Model 7600)to trace the 
wires. At frequencies below 2 MHz the Sony uses an induction coil pick up 
(as do most transistor radios do in the AM commercial broadcast band 
(500KHz to 1500 KHz) called a ferrite rod antenna. I found that at 
frequencies above 400 KHz (400,000 cycles per second) that radio broadcast 
stations and radiation from the wire itself, acting as an antenna, gave 
false readings. I found that frequencies between 200KHz and 400 kHz worked 
best for my problem and I could locate the wires to about 15 cm (6 inches) 
just by listening to the tone modulated signals from my 1 watt signal 
generator as I moved the radio across the lawns and flower beds about 15 cm 
(6 inches) above the ground. 

As I expected, maximum signal was detected with the Sony’s internal coil 
antenna normal (at right angles) to the wire and zero signal was obtained 
when the Sony’s coil was aligned with the wire. The zero signal gives a 
precise direction measurement of the wire. Longer buried 
wires might form radiating antenna at the higher frequencies so I 
suspect that for reduced interference the commercial systems work at lower 
frequencies. The pictures of the AGV antennas size indicate that they 
are probably in the same radio frequency bands that I used, although they 
could be down at 10 KHz. Audio frequencies would require large induction 
coils and might have possible interference from the power lines. Also, 
superheterodyne radio receivers are much more sensitive than audio 
amplifiers. I also noted that the AVG system uses crystal controlled 
oscillators for transmitter stability  and 100 kHz crystals are in common 
use for receiver calibration.

Best regards, your Mad Scientist
Adrian Popa